1. Field of the Invention
The present invention relates to tunable filters, and more particularly, to tunable filters that operate at radio frequencies.
2. Description of the Related Art
Wireless communications applications have increased to crowd the available spectrum and drive the need for high isolation between adjacent bands. Portability requirements of mobile communications additionally require a reduction in the size of communications equipment. Filters used in communications devices have been required to provide improved performance using smaller sized components. Efforts have been made to develop new types of resonators, new coupling structures, and new configurations to address these requirements.
Electrically tunable microwave filters have many applications in microwave systems. These applications include local multipoint distribution service (LMDS), personal communication systems (PCS), frequency hopping radio, satellite communications, and radar systems. There are three main kinds of microwave tunable filters, including mechanically, magnetically, and electrically tunable filters. Mechanically tunable filters suffer from slow tuning speed and large size. Compared to mechanically and magnetically tunable filters, electrically tunable filters have the important advantages of small size and fast tuning capability over relatively wide frequency bands. Electrically tunable filters include voltage-controlled tunable dielectric capacitor based tunable filters, and semiconductor varactor based tunable filters. Compared to semiconductor varactor based tunable filters, tunable dielectric capacitor based tunable filters have the merits of lower loss, higher power-handling, and higher IP3, especially at higher frequencies (>10 GHz).
Tunable filters offer communications service providers flexibility and scalability never before accessible. A single tunable filter can replace several fixed filters covering adjacent frequencies. This versatility provides transceiver front end RF tunability in real time applications and decreases deployment and maintenance costs through software controls and reduced component count. Also, fixed filters need to be wide band so that their count does not exceed reasonable numbers to cover the desired frequency plan. Tunable filters, however, are typically narrow band, but they can cover a larger frequency band than fixed filters by tuning the filters over a wide range. Additionally, narrowband filters at the front end are appreciated from the systems point of view, because they provide better selectivity and help reduce interference from nearby transmitters.
There are several patents that address either changing the resonant frequency of a resonator and/or changing the coupling between resonators in order to create a tunable filter. Two methods appear to predominate this prior art. One is to use a semiconductor varator diode by directly attaching it to the resonators (to ground for frequency shift and across resonators for coupling change). The second is to use a dielectric varactor (a ferro-electric material whose dielectric constant varies with an applied voltage.) This type of varactor is typically used as a layer between the conductors of a microstrip or strip line type of filter (combline, hairpin, interdigital), a substrate and a ground plane.
Dielectric varactors require large bias voltages and are likely to be somewhat lossy. Examples of these patents that utilize dielectric varactors include: U.S. Pat. No. 7,148,770, entitled “Electrically Tunable Bandpass Filters,” issued to Toncich; U.S. Pat. No. 6,525,630, entitled “Microstrip Tunable Filters Tuned by Dielectric Varactors,” issued to Zhu et al.; U.S. Pat. No. 6,686,817, entitled “Electronic Tunable Filters with Dielectric Varactors,” issued to Zhu et al.; and, U.S. Pat. No. 6,216,020, entitled “Localized Electrical Fine Tuning of Passive Microwave and Radio Frequency Devices”, issued to Findikoglu.
The disadvantage of the diode varactors for changing the coupling is that the coupling capacitances are so low that they are challenging to use and since they have to be connected directly to the resonators the biasing circuit affects the performance. Examples of these patents that utilize semiconductor diode varactors include: U.S. Pat. No. 7,113,059, entitled “Variable-Frequency High Frequency Filter,” issued to Asamura; U.S. Pat. No. 6,717,491, entitled “Hairpin Microstrip Line Electrically Tunable Filters,” issued to Liang et al.; and, U.S. Pat. No. 4,835,499, entitled “Voltage Tunable Bandpass Filter,” issued to Pickett.